Announce widget added for accessibility

This change adds a custom announcement for accessibility when a new
widget is added in the Glanceable Hub.

Test: manually verified announcement for both scrolling and
      non-scrolling cases
Test: atest CommunalEditModeViewModelTest
Fix: 348281765
Flag: com.android.systemui.communal_hub
Change-Id: I1dca6f470ca66e13835dd601916b3fc4c9fcfdd7